The pilot was manoeuvring the helicopter, during a stock shooting operation, at a low speed, a low altitude and a high power setting when the tail rotor peddles jammed. The aircraft began to turn, uncontrollably, to the right. When the pilot reduced airspeed, in an attempt to avoid a tree, the aircraft touched down heavily. A spent cartridge case was found jammed in the operating levers of the left hand side (pilot's) tail rotor peddles. The helicopter had been checked, prior to DEPARTURE, for loose objects and none were found. The spent cartridge probably fell inside the cabin of the helicopter during that day's shoot. The shooter did not have a cartridge case catcher fitted to his gun. There was no rubber boot, around the base of the tail rotor peddles, which would prevent foreign objects falling through the hole, at the base of the peddles, and interferring with the aircraft's controls. The proximity of the aircraft to the ground and the trees, a typical hazard during stock shooting operations, prevented the pilot from taking recovery action before the aircraft collided with the ground.